Process for treating a solid-liquid mixture

ABSTRACT

A process for treating a solid-liquid mixture by cavitation has been developed to decompose at least some contaminant associated with the solid particles, the contaminant either being adsorbed into the pores of the solid or onto the surface of the solid particles. The process includes the step of subjecting the mixture to cavitation such that a portion of the contaminant is chemically decomposed. Typically the chemical decomposition occurs at the surface of the solid particles, although the process can also occur to some extent within the poses near the surface of the solid material being treated. Typically the cavitation process is an ultrasonic treatment step, although other cavitation processes are applicable, for example high shear mixing. The cavitation effect is capable of achieving physico-chemical changes at the particle surfaces. The localised high temperatures on bubble collapse (as high as 5000K) can decompose contaminant substances such as PCB and other hazardous materials including polybrominated biphenyl (PBB), organochloride and organophosphate compounds, pesticides and the like. One of the advantages of the treatment process is that the decomposition products are quenched quickly to the temperature of the bulk fluid (at, for example, 50° C.) which avoids the reformation of the PCB or the formation of undesirable side reaction products such as dioxins.

FIELD OF THE INVENTION

[0001] The present invention relates to a process for the decompositionof contaminant substances. The method can be applied to decontaminatesoils and other substrates containing polychlorinated biphenyl (PCB)compounds in domestic, municipal or industrial applications and willprimarily be described with reference to this context. It should beremembered, however, that the invention has broader use in thedecomposition of all manner of hazardous materials includingpolybrominated biphenyl (PBB), organochlorides and organophosphatecompounds, pesticides and the like.

BACKGROUND ART

[0002] Polychlorinated biphenyls (PCB compounds) were first discoveredto be environmental pollutants in 1966. They have been round throughoutthe world in water, solid sediments, and bird and fish tissue. There aresome 209 different PCB compounds available, made by substituting from 1to 10 chlorine atoms onto a biphenyl aromatic structure. PCB compoundshave very high chemical, thermal and biological stability, and a low,water solubility and vapour pressure. While these useful propertiescontributed to their widespread use, those same properties allowed thesecompounds to be accumulated in the environment.

[0003] The manufacture of PCB compounds was discontinued in the UnitedStates in 1979, although these compounds continue to enter theenvironment from discarded electrical equipment, etc. PCB concentrationsof 1-2 ppm are normally the desired maxima, and levels of 10-50 ppm inagricultural soils, clays or marine sediments are considered hazardous.The dense and hydrophobic nature of PCB compounds ensures that theiraccumulation in river sediment is commonplace, leading tobioaccumulation in bottom dwellers and fish thus leading to entry intothe human food chain. PCB compounds can reduce human disease resistance,and increase the incidence of rashes, liver ailments and headaches.Similarly, pesticides can have serious health effects on humans andanimals.

[0004] Numerous investigations of ways to degrade PCB compounds andpesticides have been carried out. At present there are no widelyaccepted methods for the large scale remediation of water or soilscontaminated with PCB compounds or pesticides. The decomposition of PCBand organochloride compounds can be effected by high temperatureincineration at a typical temperature of 1300° C. but the gaseousproducts must be quenched quickly to avoid the reformation of the PCB orthe formation of undesirable side reaction products such as dioxins at800-900° C. Such a process is complicated and with variable or uncertainoutcomes. Biodegradation with microorganisms and chemical treatment aremethods which require lengthy treatment periods. Photocatalytic (UV)degration of contaminated soil-water systems has also been tried but isalso slow.

[0005] Ultrasound is known in the art for inducing chemical reactionprocesses in liquids, a field known as sonochemistry. The propagation ofultrasonic waves in a liquid generates cavitation bubbles. These bubblesimplode and produce micro-regions of extreme conditions. Estimatedtemperatures within these micro-regions range from 2000-5000K in aqueoussolution. In U.S. Pat. No. 5,498,431 a process is described fordecontaminating particulate surfaces by the use of ultrasound to firstlyrelease mycotoxins from the particulates into an aqueous liquid followedby a chemical reaction breakdown of the contaminants by ultrasound whenin the liquid. The cavitation from the ultrasound leads to asonochemical breakdown reaction of the mycotoxin contaminants when inthe aqueous liquid. In WO96/20784 a method of chemical reactioncatalysis in a liquid is described which is facilitated by ultrasoniccavitation. The cavitation is aided by the presence of solid particlesas a surface for ‘seeding’ the cavitation bubbles prior to theirseparation from the solid particles whereupon the bubbles cavitate(implode) in the liquid medium.

[0006] Ultrasound has been used to decompose PCB compounds that aredissolved in an aqueous solution. However, because of their lowsolubility, the concentration of PCB compounds in aqueous solution isvery low when compared with that found adsorbed onto solids, riversediment and the like, so that such an aqueous treatment technique islargely ineffective.

[0007] It is to be understood that, if any prior art publication isreferred to herein, such reference does not constitute an admission thatthe publication forms a part of the common general knowledge in the art,in Australia or any other country.

SUMMARY OF THE INVENTION

[0008] In a first aspect the present invention provides a process fortreating a mixture of a solid and a liquid to decompose a contaminantassociated with the solid, said process including the step of subjectingthe mixture to cavitation wherein at least a portion of the contaminantis chemically decomposed, the chemical decomposition occurring at ornear a surface of the solid.

[0009] Such a process can provide an improved technique for thedecomposition of contaminant substances by providing for localised hightemperatures followed immediately by a quenching of the decompositionproducts (ie. by the liquid) thereby avoiding the reformation of thesubstance or the formation of undesirable side reaction products atcertain temperatures. The technique can effectively treat contaminatedsolid particles at their surface where the concentration of contaminantsis at its highest when compared with the aqueous phase.

[0010] In the prior art processes for the chemical decomposition of acontaminant by the use of cavitation, a physical separation of thecontaminant from a substrate material into a liquid occurs so thatsonochemical reactions can occur in the liquid. In the present process aphysical separation of a contaminant from a substrate into a surroundingliquid is not required and the contaminant is present at or near thesurface of the solid. U.S. Pat. No. 5,498,431 and WO96/20784 discloseonly that chemical decomposition occurs in the surrounding liquid.

[0011] Preferably the cavitation process is effected by an ultrasonictreatment process using ultrasonic source equipment such as ultrasonicplates, probes, baths or other chambers.

[0012] Preferably the process also includes the step of mixing the solidand liquid whereby the solid is substantially suspended in the liquid toincrease exposure of the mixture to cavitation.

[0013] Preferably the solid includes mineral and/or organic matter. Mostpreferably the solid includes one or more materials such as silica,clay, carbonaceous material, activated carbon or calcium carbonate.

[0014] In a second aspect the present invention provides a process fortreating a mixture of a solid and a liquid to decompose a contaminantassociated with the solid, said process including the step of subjectingthe mixture to cavitation wherein at least a portion of the contaminantis chemically decomposed and wherein at least some of the solid servesto catalyse the decomposition.

[0015] Preferably in this second aspect the chemical decompositionoccurs at or near a surface of the solid.

[0016] Preferably the other process steps of the second aspect are asdefined in the first aspect.

[0017] In a third aspect the present invention provides a process fortreating and decomposing a contaminant in a liquid which includes thecontaminant, the process including the steps of:

[0018] adsorbing the contaminant on a solid; and

[0019] subjecting a mixture of at least some of the solid and at leastsome of the liquid to cavitation such that at least a portion of thecontaminant associated with the solid is chemically decomposed at ornear a surface of the solid.

[0020] Preferably the process steps of the third aspect are as definedin the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Notwithstanding any other forms which may fall within the scopeof the present invention, a preferred form of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

[0022]FIG. 1 shows some experimental results for the decomposition of aPCB located on a silica particulate substrate as a function of time;following the treatment of aqueous particulate suspensions in accordancewith an embodiment of the invention.

[0023]FIG. 2 shows some other experimental results for the decompositionof a PCB at a higher initial concentration located on a silicaparticulate substrate as a function of time; following the treatment ofaqueous particulate suspensions in accordance with an embodiment of theinvention.

[0024]FIG. 3 shows some other experimental results for the decompositionof a PCB located on a silica particulate substrate as a function oftime; following the treatment of aqueous particulate suspensions inaccordance with an embodiment of the invention.

[0025]FIG. 4 shows some other experimental results for the decompositionof a PCB located on a calcium carbonate particulate substrate as afunction of time; following the treatment of aqueous particulatesuspensions in accordance with an embodiment of the invention.

[0026]FIG. 5 shows some experimental results for the decomposition of apesticide (Chlordane) located on a silica particulate substrate as afunction of time; following the treatment of aqueous particulatesuspensions in accordance with an embodiment of the invention.

[0027]FIG. 6 shows some experimental results for the decomposition of apesticide (DDT) located on a silica particulate substrate as a functionof time; following the treatment of aqueous particulate suspensions inaccordance with an embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION

[0028] A process for treating a solid-liquid mixture by cavitation hasbeen developed to decompose at least some contaminant associated withthe solid particles, the contaminant either being adsorbed into thepores of the solid or onto the surface of the solid particles.

[0029] The process includes the step of subjecting the mixture tocavitation such that a portion of the contaminant is chemicallydecomposed. Typically the chemical decomposition occurs at the surfaceof the solid particles, although the process can also occur to someextent within the pores near the surface of the solid material beingtreated. In the preferred embodiment the cavitation process is anultrasonic treatment, step, although other cavitation processes areapplicable, for example high shear mixing.

[0030] Under the influence of ultrasound, the formation of a vapourbubble (as distinct from one formed from dissolved gases) occurs whenstress in the liquid (due to the negative pressure produced during theexpansion cycle of a sound wave) exceeds the tensile strength of theliquid. The stress at a solid-liquid boundary, due to the presence of anultrasonic field, is much greater than in the bulk of a liquid. Thelikelihood of a vapour bubble forming at the solid-liquid boundary isaround twice that in the body of liquid. This applies to both solidparticle suspensions as well as to the walls of a vessel. The smallerparticles are more likely to support vapour bubble nuclei because oftheir high surface area and surface free energy.

[0031] Large amounts of energy are released from the cavitation collapseof vapour bubbles at or near the surface of the solids. The manner ofthe collapse near a surface takes the form of a high velocity jetdirected at that surface. This effect is capable of achievingphysico-chemical changes at the particle surfaces.

[0032] The inventor has surprisingly discovered that the localised hightemperatures on bubble collapse (as high as 5000K) can decomposecontaminant substances such as PCB and other hazardous materialsincluding polybrominated biphenyls (PBB), organochloride andorganophosphate compounds, pesticides and the like. One of theadvantages of the treatment process is that the decomposition productsare quenched quickly to the temperature of the bulk fluid (at, forexample, 50° C.) which avoids the reformation of the PCB or theformation of undesirable side reaction products such as dioxins.

[0033] In the preferred embodiment the solid-liquid mixture beingtreated by such a process can also be mixed by means of an impeller orsimilar stirring device in a mixing vessel to cause the solid-liquidmixture to become substantially suspended. This can maximise theexposure of the particle surfaces in the mixture to cavitation. It isalso possible that the mixture can be stirred simultaneously withinsonation or as separate steps.

[0034] Typically the solid particles are mineral and/or organic matterfor example silica (sand), calcium carbonate, carbonaceous matterincluding activated carbon, clay or soils and sediments containingorganics and/or mixtures thereof.

[0035] The role of the solid substrate can also be to catalyse thedecomposition depending upon the material chosen. The substrate can infact catalyse the rate and the extent of the decomposition reaction.Such substrate materials may include titanium dioxide, for example (aknown photocatalytic material).

[0036] The porosity of the substrate can also influence the quantity ofPCB available for surface or near surface reaction. Very adsorptive orporous substrates such as activated carbon or charcoal can adsorb alarge quantity of a contaminant substance and make this materialavailable at the surface for reaction.

[0037] The source of the ultrasound can be any suitable device which canbe used to deliver sound waves of sufficient power and intensity,typically an ultrasonic bath, plate or probe source.

[0038] In use the process can provide an improved technique for thedecomposition of PCB and other hazardous substances by providing alocalised high temperatures followed immediately by a quenching of thedecomposition products thereby avoiding the reformation of the substanceor the formation of undesirable side reaction products. The techniquecan effectively treat contaminated solid particles by a surface reactionwhich is where the concentration of contaminants is highest whencompared with the aqueous phase.

[0039] The process can also be applied to situations where acontaminated liquid flow requires effective treatment. Normally the useof ultrasound to treat low levels of PCB or pesticides etc whendissolved in a liquid stream is an ineffective process. Large volumes offluid having a low concentration of contaminant are not able to beefficiently processed. As an alternative, the PCB or other contaminantcan be adsorbed onto a solid substrate and the substrate then subjectedto a cavitation step to effect the chemical decomposition of the muchmore concentrated contaminant. If a high surface area reusable materialsuch as activated carbon or clay solids was used, the process can berepeatedly applied to a liquid stream using the same recycled solidmaterials.

[0040] Whilst the invention has been described with reference to anumber of preferred embodiments it should be appreciated that theinvention can be embodied in many other forms.

[0041] The following experimental, examples show the reduction ofadsorbed PCB concentrations on solid particles following ultrasonictreatment in an aqueous pulp.

[0042] Calcium carbonate and silica (sand) solids were mixed separatelywith a PCB compound which had been separately dissolved in acetone toform a solution. The PCB compound selected was available under the tradename ARACLOR 1260. The mixture was then evaporated to dryness and thePCB then became surface adsorbed onto the solids. A 100 g quantity ofthese solids and an equivalent weight of water were then agitated toproduce an aqueous slurry batches of which were experimentally subjectedto ultrasound at a frequency of 20 kHz and a power input of 170W. Theresidual PCB remaining on the solids as a function of time was measuredby gas chromatography.

[0043]FIG. 1 depicts the reduction in measured PCB on silica solidsfollowing extended periods of sonication up to 60 minutes. The initialconcentration of PCB was around 8 ppm and was reduced to around 2 ppmafter 60 minutes, representing around 75% decomposition.

[0044]FIG. 2 depicts the reduction in measured PCB on silica solidsfollowing extended periods of sonication up to 60 minutes. The initialconcentration of PCB was above 50 ppm, and was reduced to around 24 ppmafter 60 minutes representing more than 50% decomposition.

[0045]FIG. 3 depicts the reduction in measured PCB on silica solidsfollowing periods, of sonication up to 10 minutes. The initialconcentration of PCB was around 60 ppm and was reduced to around 35 ppmafter 10 minutes representing around 45% decomposition.

[0046]FIG. 4 depicts the reduction in measured PCB on calcium carbonatesolids following periods of sonication up to 10 minutes. The initialconcentration of PCB was around 100 ppm and was reduced to around 65 ppmafter 10 minutes representing around 35% decomposition.

[0047] The following experimental example shows the reduction ofadsorbed DDT and chlordane concentrations on solid particles followingultrasonic treatment in an aqueous pulp.

[0048] Silica (sand) solids were mixed separately with DDT and chlordanewhich had been separately dissolved in acetone to form a solution. Eachmixture was then evaporated to dryness, the respective pesticides thenbeing surface adsorbed onto the solids. A 100 g quantity of these solidsand an equivalent weight of water were then agitated to produce anaqueous slurry batches of which were experimentally subjected toultrasound at a frequency of 20 kHz and a power input of 170W. Theresidual DDT and chlordane remaining on the solids as a function of timewas measured by gas chromatography.

[0049]FIG. 5 depicts the reduction in measured chlordane and FIG. 6 thereduction in measured DDT concentration on silica solids followingextended periods of sonication up to 30 minutes. The initialconcentration of DDT was around 715 ppm and was reduced to around 185ppm after 30 minutes, representing around 74% decomposition. In the caseof the chlordane, the initial concentration of chlordane was around 715ppm and was reduced to around 270 ppm after 30 minutes, representingaround 62% decomposition.

1. A process for treating a mixture of a solid and a liquid to decomposea contaminant associated with the solid, said process including the stepof subjecting the mixture to cavitation wherein at least a portion ofthe contaminant is chemically decomposed, the chemical decompositionoccurring at or near a surface of the solid.
 2. A process for treating amixture as claimed in claim 1 wherein the cavitation process is effectedby an ultrasonic treatment process.
 3. A process for treating a mixtureas claimed in claim 1 or claim 2 also including the step of mixing thesolid and liquid whereby the solid is substantially suspended in theliquid to increase exposure of the mixture to cavitation.
 4. A processfor treating a mixture as claimed in any one of the preceding claimswherein the solid includes mineral and/or organic matter.
 5. A processfor treating a mixture as claimed in any one of the previous claimswherein the solid includes one or more materials such as silica, clay,carbonaceous material, activated carbon or calcium carbonate.
 6. Aprocess for treating a mixture of a solid and a liquid to decompose acontaminant associated with the solid, said process including the stepof subjecting the mixture to cavitation wherein at least a portion ofthe contaminant is chemically decomposed and wherein at least some ofthe solid serves to catalyse the decomposition.
 7. A process as claimedin claim 6 wherein the chemical decomposition occurs at or near thesurface of the solid.
 8. A process as claimed in claim 6 or claim 7wherein the process steps are as defined in any one of claims 2 to
 5. 9.A process for treating and decomposing a contaminant in a liquid whichincludes the contaminant, the process including the steps of: adsorbingthe contaminant on a solid; subjecting a mixture of at least some of thesolid and at least some of the liquid to cavitation such that at least aportion of the contaminant associated with the solid is chemicallydecomposed at or near a surface of the solid.
 10. A process as claimedin claim 9 wherein the process steps are as defined in any one of claims2 to
 5. 11. A process for treating a mixture of a solid and a liquid,said process being substantially as herein described with reference tothe accompanying examples.
 12. A process for treating and decomposing acontaminant in a liquid flow which includes the contaminant, saidprocess being substantially as herein described with reference to theaccompanying examples.